Transcript slides - Temple University

The Relational Model
Content based on Chapter 3
Database Management Systems, (Third Edition),
by Raghu Ramakrishnan and Johannes Gehrke.
McGraw Hill, 2003
“The most important motivation for the
research work that resulted in the
relational model was the objective of
providing a sharp and clear boundary
between the logical and physical aspects
of database management.” ―E. F. Codd
Why Study the Relational Model?

Most widely used model.


“Legacy systems” in older models


Vendors: IBM, Informix, Microsoft, Oracle,
Sybase, etc.
E.G., IBM’s IMS
Recent competitor: object-oriented and
NoSQL models


ObjectStore, Versant, Ontos
A synthesis emerging: object-relational model
• Informix Universal Server, UniSQL, O2, Oracle, DB2
Relational Database: Definitions
Relational database: a set of relations
 Relation: made up of 2 parts:



Instance : a table, with rows and columns.
#Rows = cardinality, #fields = degree / arity.
Schema : specifies name of relation, plus name and
type of each column.
• E.G. Students(sid: string, name: string, login: string,
age: integer, gpa: real).

Can think of a relation as a set of rows or tuples
(i.e., all rows are distinct).
Example Instance of Students Relation
sid
53666
53688
53650
name
login
Jones jones@cs
Smith smith@eecs
Smith smith@math
age
18
18
19
gpa
3.4
3.2
3.8

Cardinality = 3, degree = 5, all rows distinct

Do all columns in a relation instance have to
be distinct?
Another Example
Relational Query Languages
A major strength of the relational model:
supports simple, powerful querying of data.
 Queries can be written intuitively (experience
says differently!), and the DBMS is
responsible for efficient evaluation.



The key: precise semantics for relational queries.
Allows the optimizer to extensively re-order
operations, and still ensure that the answer does
not change.
The SQL Query Language
Developed by IBM (system R) in the 1970s
 Need for a standard since it is used by many
vendors
 Standards:









SQL-86
SQL-89 (minor revision)
SQL-92 (major revision)
SQL-99 (major extensions)
SQL-2003 (added XML-related features)
SQL-2006 (extends XML-related features)
SQL-2008 (minor additions)
SQL-2011 (more minor additions, current)
SQL Components

SQL - Structured Query Language
 This sounds odd, isn’t it?
• Structured Query Language query langue.
DML – Data Manipulation Language
 DDL - Data Definition Language

 E.g., CREATE, ALTER.

DCL – Data Control Language
 E.g., GRANT, REVOKE.
The SQL Query Language

To find all 18 year old students, we can write:
SELECT *
FROM Students S
WHERE S.age=18
sid
name
53666 Jones
login
jones@cs
age gpa
18
3.4
53688 Smith smith@ee 18
3.2
•To find just names and logins, replace the first line:
SELECT S.name, S.login
Querying Multiple Relations

What does the
following query
compute?
SELECT S.name, E.cid
FROM Students S, Enrolled E
WHERE S.sid=E.sid AND E.grade=“A”
Given the following instances
of Enrolled and Students:
sid
53666
53688
53650
name
login
age gpa
Jones jones@cs
18 3.4
Smith smith@eecs 18 3.2
Smith smith@math 19 3.8
sid
53831
53831
53650
53666
cid
grade
Carnatic101
C
Reggae203
B
Topology112
A
History105
B
we get:
S.name E.cid
Smith
Topology112
Creating Relations in SQL
Creates the Students
CREATE TABLE Students
(sid: CHAR(20),
relation. Observe that the
name: CHAR(20),
type (domain) of each field
login: CHAR(10),
is specified, and enforced by
age: INTEGER,
the DBMS whenever tuples
gpa: REAL)
are added or modified.
 As another example, the
CREATE TABLE Enrolled
Enrolled table holds
(sid: CHAR(20),
information about courses
cid: CHAR(20),
that students take.
grade: CHAR(2))

Destroying and Altering Relations
DROP TABLE Students

Destroys the relation Students. The schema
information and the tuples are deleted.
ALTER TABLE Students
ADD COLUMN firstYear: integer

The schema of Students is altered by adding a
new field
 Every tuple in the current instance is extended
with a null value in the new field.
Having Fun with SQL
Source: http://xkcd.com/327/
Adding and Deleting Tuples

Can insert a single tuple using:
INSERT INTO Students (sid, name, login, age, gpa)
VALUES (53688, ‘Smith’, ‘smith@ee’, 18, 3.2)

Can delete all tuples satisfying some
condition (e.g., name = Smith):
DELETE
FROM Students S
WHERE S.name = ‘Smith’
* Powerful variants of these commands are available; more later!
Integrity Constraints (ICs)

IC: condition that must be true for any instance
of the database; e.g., domain constraints.



A legal instance of a relation is one that satisfies
all specified ICs.


ICs are specified when schema is defined.
ICs are checked when relations are modified.
DBMS should not allow illegal instances.
If the DBMS checks ICs, stored data is more
faithful to real-world meaning.

Avoids data entry errors, too!
Primary Key Constraints

A set of fields is a key for a relation if :
1. No two distinct tuples can have same values in all
key fields, and
2. This is not true for any subset of the key.
 If Part 2 false? Then, a superkey.
 If there’s more than one key for a relation, one of
the keys is chosen (by DBA) to be the primary key.

E.g., sid is a key for Students. (What about
name?) The set {sid, gpa} is a superkey.
Primary and Candidate Keys in SQL



Possibly many candidate keys (specified using
UNIQUE), one of which is chosen as the primary key.
“For a given student and course, CREATE TABLE Enrolled
(sid CHAR(20)
there is a single grade.” vs.
cid CHAR(20),
“Students can take only one
grade CHAR(2),
course, and receive a single grade
PRIMARY KEY (sid,cid) )
for that course; further, no two
CREATE TABLE Enrolled
students in a course receive the
(sid CHAR(20)
same grade.”
cid CHAR(20),
Used carelessly, an IC can prevent
grade CHAR(2),
the storage of database instances
PRIMARY KEY (sid),
that arise in practice!
UNIQUE (cid, grade) )
Foreign Keys, Referential Integrity
Foreign key : Set of fields in one relation that is used
to `refer’ to a tuple in another relation. (Must
correspond to primary key of the second relation.)
Like a `logical pointer’.
 E.g. sid is a foreign key referring to Students:




Enrolled(sid: string, cid: string, grade: string)
If all foreign key constraints are enforced, referential
integrity is achieved, i.e., no dangling references.
Can you name a data model w/o referential integrity?
• Links in HTML!
Foreign Keys in SQL

Only students listed in the Students relation should
be allowed to enroll for courses.
CREATE TABLE Enrolled
(sid CHAR(20), cid CHAR(20), grade CHAR(2),
PRIMARY KEY (sid,cid),
FOREIGN KEY (sid) REFERENCES Students )
Enrolled
sid
53666
53666
53650
53666
cid
grade
Carnatic101
C
Reggae203
B
Topology112
A
History105
B
Students
sid
53666
53688
53650
name
login
Jones jones@cs
Smith smith@eecs
Smith smith@math
age
18
18
19
gpa
3.4
3.2
3.8
Enforcing Referential Integrity
Consider Students and Enrolled; sid in Enrolled is a
foreign key that references Students.
 What should be done if an Enrolled tuple with a
non-existent student id is inserted?

 (Reject it!)

What should be done if a Students tuple is deleted?





Also delete all Enrolled tuples that refer to it.
Disallow deletion of a Students tuple that is referred to.
Set sid in Enrolled tuples that refer to it to a default sid.
(In SQL, also: Set sid in Enrolled tuples that refer to it to a
special value null, denoting `unknown’ or `inapplicable’.)
Similar if primary key of Students tuple is updated.
Example Deletion Strategies
sid
53666
53688
53650
name
login
age gpa
Jones jones@cs
18 3.4
Smith smith@eecs 18 3.2
Smith smith@math 19 3.8
sid
53831
53831
53650
53666
cid
grade
Carnatic101
C
Reggae203
B
Topology112
A
History105
B
sid
53831
53831
53650
53666
NULL
cid
grade
Carnatic101
C
Reggae203
B
Topology112
A
History105
B
Example Update Strategies
sid
53666
55666
53688
53650
name
login
age gpa
Jones jones@cs
18 3.4
Smith smith@eecs 18 3.2
Smith smith@math 19 3.8
sid
53831
53831
53650
53666
NULL
cid
grade
Carnatic101
C
Reggae203
B
Topology112
A
History105
B
sid
53831
53831
53650
53666
99999
cid
grade
Carnatic101
C
Reggae203
B
Topology112
A
History105
B
Referential Integrity in SQL

SQL/92 onwards support CREATE TABLE Enrolled
all 4 options on deletes
(sid CHAR(20),
and updates.
cid CHAR(20),
grade CHAR(2),
 Default is NO ACTION
PRIMARY KEY (sid,cid),
(delete/update is rejected)
FOREIGN KEY (sid)
 CASCADE (also delete
REFERENCES Students
all tuples that refer to
ON DELETE CASCADE
deleted tuple)
ON UPDATE SET DEFAULT )
 SET NULL / SET DEFAULT
(sets foreign key value
of referencing tuple)
Where do ICs Come From?
ICs are based upon the semantics of the realworld enterprise that is being described in the
database relations.
 We can check a database instance to see if an
IC is violated, but we can NEVER infer that
an IC is true by looking at an instance.




An IC is a statement about all possible instances!
From example, we know name is not a key, but the
assertion that sid is a key is given to us.
Key and foreign key ICs are the most
common; more general ICs supported too.
Views

A view is just a relation, but we store a
definition, rather than a set of tuples.
CREATE VIEW YoungActiveStudents (name, grade)
AS SELECT S.name, E.grade
FROM Students S, Enrolled E
WHERE S.sid = E.sid and S.age<21

Views can be dropped using the DROP VIEW command.

How to handle DROP TABLE if there’s a view on the table?
• DROP TABLE command has options to let the user specify
this.
Views and Security

Views can be used to present necessary
information (or a summary), while hiding
details in underlying relation(s).

Given YoungStudents view, but not Students or
Enrolled, we can find students who are enrolled,
but not the cid’s of the courses they are enrolled in.
Relational Model: Summary



A tabular representation of data.
Simple and intuitive, currently the most widely used.
Integrity constraints can be specified by the DBA,
based on application semantics. DBMS checks for
violations.



Two important ICs: primary and foreign keys
In addition, we always have domain constraints.
Powerful and natural query languages exist.
Key Concepts